1. Field of the Invention
This invention relates to control of the reels of a recording and reproducing device which drives a tape by a capstan and more particularly to optimal control of tape tension between the reels by detecting moments of inertia of the reels.
2. Description of Related Art
For control of the reels of a magnetic recording and reproducing device, it is required to move a magnetic tape at constant speed and tension. Therefore, it is necessary to control the reels according to the moments of inertia of the reels including the tape thereon by taking into consideration diameters of the wound tape on the reels, which change every moment as the tape runs.
In prior art devices as disclosed in Japanese Patent Application Laid-Open No. 56-127957, the tape radii on the reels are detected, and then, a moment of inertia of only the tape on each reel is found, and also a moment of inertia of each reel including the tape thereon is obtained by adding a fixed value corresponding to a moment of inertia of the empty reel to the moment of inertia of only the tape on the reel. The reels are controlled according to the thus obtained moments of inertia of the reels with the tape thereon.
In the above-mentioned prior art, when there are various kinds of reels usable with the device and the moments of inertia of individual reels differ depending on their types, the moments of inertia of the reels cannot be detected accurately, resulting in a great fluctuation in tape tension and making it impossible to run a tape stably. For a VTR using a 1-inch-wide tape for broadcast, for example, there are so large a variety of reels loadable on the VTR, ranging from 6.5 to 14 inches in diameter that the greatest difference in the moment of inertia among the reels is as large as more than twenty times. FIG. 2 is a graph showing the moment of inertia of reel including a tape thereon versus tape radius. It will be understood that when the diameter of a wound tape is small (say, so small as to be close to the minimum diameter r.sub.0), it is difficult to accurately find a moment of inertia of a reel with a tape thereon from the tape radius.